Light emitting diodes and laser diodes have attained considerable significance as light sources in measurement technology and for display purposes due to their low energy requirements, their small dimensions, their robustness, their modulation ability and last but not least due to their favorable price. For certain applications, it is a disadvantage that the optical output power of these light sources is dependent on their temperature; it decreases with a rising temperature. In measurement technology, this covers all applications where a quantity is measured via the transmission capability of any optical transmission distance. Examples are methods for measuring the turbidity or color of solids, liquids or gases, and especially fiber-optical methods for measuring the various quantities, where a light conducting fiber changes its optical properties under the influence of the measured quantity. Even in applications, where light emitting diodes are used solely for display purposes, the fulfillment of customer specifications or especially of legal requirements may require special measures to compensate the influence of temperature changes. In traffic lights, light emitting diodes can enable significant energy savings, for example; however, the temperature pattern of some types suitable for this purpose and the operating temperature range is so great that the brightness stipulated by the law could not be met without proper temperature compensation.
Basically, two methods are used for light emitting or laser diodes to maintain a constant optical output power. First, they could be controlled. However, the results of this method are not as satisfactory as would be intuitively expected because the brightness measurement that is required for this control would have to take place under particularly unfavorable conditions. First, a certain portion of the emitted light would have to be diverted and would need to be kept constant, which is already a first source of error. Because this light would later not be available, one would naturally attempt to make do with as little of it as possible. This is accompanied by a decrease in the relative measurement accuracy. In addition, the controlled light may often be needed in an illuminated environment, where naturally external light may get to the measurement receiver. The influence of such external light is greater the smaller the portion of the light is that is diverted for control purposes. And in addition to all that, the measurement receiver itself often exhibits a temperature dependency and is often additionally heated up by the energy loss at the light source. The same applies to the geometry of the transmission distance between light source and measurement receiver.
One can expect better results, when the temperature of the light or laser diode is maintained at the same level. For this purpose, the component or the chip is mounted on a thermoelectric cooling element together with a temperature sensor, and the temperature measured by the temperature sensor is maintained at a consistently low level using a controller, which in turn increases the light yield. One trusts that the thermal influence of the cooling element on the light source and the temperature sensor is large in comparison to influences from the surrounding area such that a consistent temperature of the light source can be inferred from a consistent temperature of the sensor. Since this is the case in good approximation, this type of light stabilization is often preferred for measurement purposes. However, it requires equipment expenditure. Light source, cooling element and temperature sensor not only need to be paid for but also assembled, often together with an additional so-called monitor photo diode that needs to be adjusted. The cooling element requires a high current at a low voltage, which is not easily provided by usual power supplies; in addition, it generates significant waste heat that must be removed. This procedure is, therefore, relatively expensive.
A third method consists of foregoing the cooling element, using a temperature sensor to measure the temperature of the light source or the surrounding temperature, and to control the light based on such a measurement. However, since light emitting or laser diode chips are relatively small, it is difficult to achieve good thermal coupling with the temperature sensor, such that its temperature, and thus, its measurement is not entirely a measure for the temperature of the light source. This is, of course, even more true when measuring the surrounding temperature. This type of stabilization is, therefore, not very precise. However, it is not very complex, can be implemented cost-effectively and provides, therefore, a practicable solution for display purposes, for example.
The aspect of the ability for modulation must always be taken into account in connection to the temperature stabilization of light emitting diodes or laser diodes. These light sources can be switched on and off in extremely short times. Thus, they not only provide significant advantages in telecommunication, but also in measurement technology, for example, incandescent lamps. For certain measurements, the latter must be equipped with a mechanical shutter, a so-called chopper to periodically shade the light and in this manner provide a means to compensate for the influence of the surrounding light. With light emitting and laser diodes, this effect can be achieved much easier and with much higher frequencies simply by interrupting the current. A significant quality feature of a method for its stabilization is, therefore, how quickly the emitted light power assumes a stable value. All methods where, initially, after switching on the light, any measured value changes and where this value is used to correct the light power, have significant disadvantages. This is the case for both, the control of the light power and also for the measurement of the chip temperature with a sensor. Since the measurement of the surrounding temperature does not provide an accuracy sufficient for measurement purposes, a stabilized light power can be well modulated only when using a thermoelectric cooling element.
Not least for reasons of good modulation capability it has already been considered several times to use the light emitting diode or laser diode itself as a temperature sensor. After all, semi-conductor diodes are well suited for this purpose; their forward voltage changes with a constant current over the temperature in a usable linearity. A measurement obtained from the diode itself would provide precisely that temperature that influences the light power and that is to be taken into account for the stabilization. Unfortunately, taking into account a changing temperature, the stabilization must be carried out by changing the diode current which should not be changed for the temperature measurement. A temperature measurement can no longer be deduced from the forward voltage alone, instead it is also affected by the current flowing at any given moment. Determining a measurement as a function of not one but two variables, in other words, the transition from a characteristic curve to a xe2x80x9ccharacteristic areaxe2x80x9d is a path that is not preferred in measurement technology.
However, it is not absolutely necessary to explicitly know the temperature of a light emitting or laser diode to be able to compensate its influence. One can already envision this with the thought that a specific current is required for each temperature resulting in a specific value for the forward voltage from the diode characteristic curve that applies at the particular temperature. Thus, each temperature results in a point in the current/voltage diagram where the light power corresponds to the required value. For a temperature range, this turns into a line. It cannot be concluded implicitly that a combination of current and forward voltage that lies on this line will inevitably lead to the required light power. This depends on whether the respective combination is distinct for the temperature or whether it can occur at another temperature as well. If one manages to determine a line of constant light power, where all points apply distinctly for the temperature and if electrical circuit can be realized that keeps the combination of current and forward voltage stable on this line, then the light power can be stabilized via the temperature without measuring one of these two quantities.
It is the principal objective of the method described here, to provide a circuit for a light emitting diode or a laser diode such that the changes in electrical behavior caused by the temperature lead to a temperature-independence of its optical output power.
Some data sheets contain characteristic fields to characterize the electrical and the optical behavior of light emitting diodes or laser diodes as a function of the temperature, where, for example, the one field shows the forward voltage and the other the optical output power each as a function of the current with the temperature as a parameter. Theoretically, all information required for the stabilization of the respective components can be obtained from such diagrams. However, for practical applications, such information is not sufficiently accurate. This is not only due to the typically coarse presentation but also due to the fact that some particular subtleties must be taken into account during a measurement. For example, it is not particularly advisable to carry out this measurement with direct current in a temperature chamber as would correspond to the typical procedure. First, the temperature measured in this manner does not correspond to the temperature of the diode chip, whose value is not important but that matters nonetheless. Thus, the measurement of forward voltage and light power would absolutely need to be performed simultaneously to ensure that the measurements occur at the same temperature. Second, in a temperature chamber not only the light source but also the optical measurement structure would be heated up, and to design the latter in a temperature-independent fashion again constitutes particular problems.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.